1. Field of the Invention
The present invention relates to a quadrupole mass spectrometer using a quadrupole mass filter as a mass analyzer operable to separate ions according to mass values (e.g., m/z (mass-to-charge ratio) values).
2. Description of the Background Art
A quadrupole mass spectrometer is designed to apply a voltage (input voltage) formed by superimposing a high-frequency (e.g., radio-frequency) voltage on a direct-current (DC) voltage, to four rod electrodes constituting a quadrupole mass filter, to allow only an ion having a mass corresponding to a value of the input voltage to selectively pass through the quadrupole mass filter and reach an ion detector. Recently, a gas chromatograph/mass spectrometer (GC/MS) and a liquid chromatograph/mass spectrometer (LC/MS) produced by combining the quadrupole mass spectrometer with respective ones of a gas chromatograph and a liquid chromatograph are widely used in various fields.
A scan measurement and a selected ion monitoring (SIM) measurement are well known as a measurement mode of the quadrupole mass spectrometer (see, for example, the following Patent Document 1). The scan measurement is configured to repetitively perform a control/processing of scanning (continuously changing) a voltage to be applied to the rod electrodes of the quadrupole mass filter, so as to scan (continuously change) a mass value for an ion to be allowed to reach to the ion detector, over a given mass range. The scan measurement shows excellent ability, particularly, in qualitative analysis for a sample containing a substance whose mass is unknown. The SIM measurement is configured to repetitively perform mass analysis for ions having ones of a plurality of mass values pre-set by a user, while sequentially changing between the plurality of mass values. The SIM measurement shows excellent ability, particularly, in quantitative analysis for a substance whose mass is known.
In the SIM measurement, when a plurality of mass values are designated as a measurement parameter by an operator, the conventional quadrupole mass spectrometer is operable to arrange the mass values in an order designated by the operator. Thus, if the operator designates the mass values in ascending order (or descending order) of mass value, an input voltage in one cycle of the SIM measurement will be changed in a staircase pattern, as shown in FIG. 9A. Otherwise, the input voltage in one cycle of the SIM measurement will be changed up and down, as shown in FIG. 9B. In such cases, the following problems occur.
During a course of changing from a certain one to a next one of the plurality of mass values, the voltage to be applied to the rod electrodes of the quadrupole mass filter is changed in a stepped manner. Such a voltage change inevitably involves the occurrence of a certain level of overshoot (or undershoot) and ringing. Thus, it is necessary to provide a waiting time-period (i.e., a settling time-period) just after the voltage change to continue until a post-change voltage becomes moderately stable, and, after an elapse of the settling time-period, perform a substantial ion detection operation for the mass value corresponding to a value of the post-change voltage. In this case, during the settling time-period, any mass analysis for components of a sample introduced from a GC or LC into an ion source is not performed. Thus, as the settling time-period becomes longer, a time interval between measurements for the same mass value in adjacent cycles becomes larger, to cause deterioration in time resolution. Although a duration of one cycle may be shortened to enhance the time resolution, it causes a reduction in ion detection time-period for each of the mass values, which leads to deterioration in sensitivity and SN ratio. In the case where the mass values are randomly set as shown in FIG. 9B, an amount of voltage change becomes larger on average, and thereby the settling time-period undesirably becomes longer.
Further, if the quadrupole mass filter is set to allow a large number of ions to pass therethrough during a transitional period where the input voltage is changed from a first value for allowing only an ion having a certain one of the mass values to selectively pass through the quadrupole mass filter, to a second value for allowing only an ion having a next one of the mass values to selectively pass through the quadrupole mass filter, an excessive amount of ions is likely to enter the ion detector to cause a risk of shortening a usable life of the ion detector. However, the conventional quadrupole mass spectrometer is not designed while taking into account the phenomenon that unwanted ions pass through the quadrupole mass filter during the change between the mass values. Thus, depending on a setting order of the mass values and/or characteristics of the quadrupole mass spectrometer itself, an excessive amount of ions is likely to reach the ion detector.
The above problems occur not only in the SIM measurement, but also in an SIM/scan alternate measurement mode configured to alternately perform the SIM measurement for a plurality of mass values and the scan measurement over a given mass range, in one cycle, and repeat the cycle (see, for example, the following Patent Document 2).    [Patent Document 1] JP 08-129001A    [Patent Document 2] JP 2000-195464A